A vehicle is steered by transmitting an operation of a steering operating means provided in a vehicle chamber such as rotation of a steering wheel to a steering mechanism provided outside of the vehicle chamber so that wheels (in general, front wheels) for steering are directed.
In recent years, a power steering apparatus wherein an actuator for helping steering including a hydraulic cylinder, an electric motor and so on are provided in a midway of the steering mechanism is arranged so that a steering burden to a driver is reduced, has spread widely. In such an apparatus, the actuator is driven on the basis of a detected result of an operating physical force applied to the steering wheel for steering, and an operation of the steering mechanism according to the rotation of the steering wheel is assisted by a generated force of the actuator.
However, in such a conventional steering apparatus for a vehicle, it is necessary to mechanically connect the steering wheel which is the steering operating means and the steering mechanism, so there arises a problem such that a position inside the vehicle chamber to which the steering wheel is provided is limited to a position where it can be connected with the steering mechanism provided outside the vehicle chamber. Moreover, even if the steering wheel is provided to the position where it can be connected to the steering mechanism, the realization of the connection requires a complicated connecting structure, and thus it prevents lightening of a vehicle and simplification of the assembly steps of the vehicle.
For example, Japanese Utility Model Application Publication No. 2-29017 (1990) discloses a steering apparatus for a vehicle which is a link-less-type power steering apparatus in order to solve the above problem. Such a steering apparatus for a vehicle is arranged so that the steering wheel is separated from the steering mechanism, and similarly to the actuator for assisting steering in the power steering apparatus, an electric motor as the actuator for steering is provided in a midway of the steering mechanism. When the electric motor is controlled by a steering angle control means composed of a microprocessor to be driven based on detected results of an operating direction and operating quantity of the steering wheel, steering is performed according to the operation of the steering wheel.
A reaction force control means having an electric motor is additionally provided to the steering wheel which is not mechanically connected to the steering mechanism. The reaction force control means controls a motor by a reaction force instruction signal outputted by the steering angle control means to drive the motor on the basis of detected results of the vehicle speed and steering angle of the steering wheel. Thereafter, the reaction force, which is increased or decreased according to increase or decrease of the vehicle speed and steering angle and directs to a neutral position, is applied to the steering wheel. As a result, the steering can be performed in the same manner as a general steering apparatus (connection-type steering apparatus) for vehicles in which the steering wheel is mechanically connected to the steering mechanism.
The link-less-type steering apparatus for a vehicle which is arranged as mentioned above is useful not only to achieve the aforementioned object such as an increase in a degree of freedom for providing the steering wheel and the lightening of a vehicle but also to develop an automobile technique in the future such as realization of a new steering operating means such as a lever and pedal instead of the steering wheel, detection of guide signs on roads, and realization of an automatic operating system according to running information such as reception of satellite information.
The reaction force control means monitors an electric current flowing in the electric motor and its time, and when detecting an overload state of the electric motor (for example, the electric current of 25A flows for 20 seconds), the reaction force control means reduces the electric current flowing in the electric motor forcibly to prevent an overheating state, and reduces the reaction force. FIG. 1 is a graph showing an example of a characteristic of a relationship between a reaction force torque and steering angle in an overload protecting time in the ordinate and abscissa. In this case, the reaction force torque which is the same as the normal time is applied until a predetermined steering angle, but when the steering angle exceeds the predetermined angle, the electric current flowing in the electric motor is restricted so that the reaction force torque does not exceed a predetermined value.
However, if the electric current is restricted so that the reaction force torque does not exceed the predetermined value when the steering angle exceeds the predetermined angle in such a manner, a driver feels that the reaction force torque is changed abruptly at the steering angle, and thus the driver may possibly feel incongruous.